I’m working with a general contactor rebuilding my house after a fire on the coast of Maine. The full basement survived and the framing has begun. I wanted a super insulated house so I dug out around the foundation and installed 4″ of XPS. We’ve decided to do the same on the outside of the walls and roof, and fill the 2×6 wall framing, and 2×10 rafters with dense cellulose and possibly a thin sealing layer of hard spray foam. We’re going to side with cedar shingles treated with bleaching oil. Right now we have the inner shell done and we’re about to start applying the foam and outer layer of sheathing. I’m planning on taping the foam, at least the outer layer, and putting more sheathing outside to nail the shingle to. The windows and doors are getting boxed out. The outer layer of sheathing will attach to the rafter tails and the overhangs applied separately.
After reading the Building Science book on cold climate construction, and spending hours on the internet, my builder and I still aren’t sure what we should do between the foam and outer layer of sheathing, and underneath the shingles. Would it be worth furring under the outer sheathing to create a drain plane or not? Should we use a draining housewrap under the shingles, or just felt or tyvek?
Any thoughts? Thanks in advance for any input.
Replies
Overhanging roof? Height of walls?
How much direct weather will the siding take? Some houses are protected from prevailing winds and have enough roof overhang that rain seldom hits the siding.
You said "the coast of Maine". Does that mean directly on the coast? As in exposed to salt water spray? I'm thinking a true marine environment would be extra hard on exterior surfaces.
Here in the moderate climate of western WA, elevation 175' +/- I am totally comfortable using 30lb felt and no rain screen under wetern red cedar siding of any type. No need to even back prime it. Done it lots and seen it plenty. But key components to longevity are roofline, building siting and proper application of siding.
Not sure what grade or type shingles you intend to use, that willmake a difference too.
wall system
Thanks for the comments. The overhang will be 20" on the eaves and 14" on the rakes. We're on the water but set back enough so there won't be direct salt spray but there will be plenty of wind driven rain. I'm planning on using clear white cedar shingles factory treated with bleaching oil.
"White Cedar"?
I'll bow out, now. Not that I'm not interested, I am, but I have never worked with white cedar. I've seen lots of references to it in magazines, but I don't know squat about it.
Hopefully someone will come along and give their opinion(s).
I'm guessing the same priciples apply regarding overhangs though. Any way you'd consider extending those? Sure exterior materials should be able to take the weather, but the less water running down the siding the better in my opinion.
If you've already got sheathing under the foam board, I don't see why you need more outside it. Instead of sheathing, what you could to is tape seal/foam the seams to create an air/vapor barrier. Then attach vertical furring for the rain screen drainage plane, then the horizontl nailers for your cedar shingles. This will give you plenty of space for the shingles to dry to both sides, while also keeping an internal drainage plane.
No tar paper or Tyvec needed.
nailers
We thought about that and decided that sheathing would go on quicker that strapping every 5", especially since we're screwing through 4" of foam. We've decided to vent between the foam and sheathing, and we're using Zip system osb so we won't use housewrap under the shingles.
We had the bright idea of integrating the wall and roof venting so the air would flow from the base of the wall up to the ridge vent, but then I wondered what would happen on a warm late winter day when warm moist air would be getting sucked in at the bottom of the walls. When it hit the underside of the still snow covered roof it might condense... I had visions of water seeping out through the screening at the bottom of my walls. So I think we'll have the wall and roof vent systems separate. We'll let the air sneak in behind the drip edge for the roof, and we'll let the wall breathe behind the frieze. All because of this paragraph in Lstiburek's book, Builder's Guide to Cold Climates.
"In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to flush heat away trapped due to the insulating value of the snow. (The snow becomes and insulating blanket). This approach creates a vented-unvented hybrid roof assembly."